TAB semiconductor devices are devices which use tape-automated bonding (TAB) techniques and components. TAB devices are becoming increasingly popular because TAB tapes enable conductive leads formed on the tape to be very closely spaced. Closely spaced leads permit semiconductor manufacturers to keep device size to a minimum, a goal manufacturers are continually trying to achieve.
FIG. 1 illustrates a conventional TAB tape 10 which is representative of those used in semiconductor device fabrication. TAB tape 10 is formed of a carrier film 12 which is typically a thin polymer, for example a polyimide. Carrier film 12 is provided with alignment holes 14 which are used to align the film in various fabrication tools and with sprocket holes 16 which may be used to advance the film through these tools. A component receiving area 18 is also provided in the film for placement of an electrical component such as an integrated circuit (not shown). On carrier film 12, a plurality of electrical leads 20 extend outward from each side of component receiving area 18. The electrical leads are typically formed from a thin copper film which is either plated on or adhesively bonded to carrier film 12. Each lead has an inner lead portion 22 which is to be electrically coupled to bonding pads of an electronic component (not shown) during inner lead bonding (ILB), and an outer lead portion 24 which is to be electrically coupled to external circuitry (not shown) during outer lead bonding (OLB). Outer lead portions 24 may be coupled to a conventional lead frame, in which case leads 20 are completely internal to a packaged semiconductor device, or the outer lead portions may be external to a packaged device and be electrically coupled to, for example, a printed circuit board. In other applications, for example in flip TAB technology, inner lead portions 22 are cut from carrier film 12 such that the severed ends of the inner leads are bonded to external circuitry and the outer lead portions are not used.
A common feature of existing TAB tape structures, and of other types of lead frames, is that lead pitch is most narrow near the component receiving area and progressively gets wider as the leads extend outward. Lead pitch refers to the closeness of the leads and is usually defined as the distance from the center of one lead to the center of an adjacent lead. Another common feature of many TAB tape structures is that there are often rather large areas of the carrier film which are bare, or in other words have no leads formed thereon. With reference to FIG. 1, these bare areas are depicted as regions A, B, C, and D within carrier film 12. Regions A, B, C, and D are created as a result of having leads 20 extend outward from each of four sides of component receiving area 18. Because the lead pitch changes as the leads extend outward from the component receiving area, regions A, B, C, and D usually approximate a diamond-shaped appearance at corners adjacent to component receiving area 18. In other cases, the pitch of the inner and outer lead portions is the same (in other words the leads do not "fan-out") in which case regions A, B, C, and D may not be diamond-shaped.
The presence of bare carrier film regions A, B, C, and D creates a substantial problem in fabricating reliable semiconductor devices which use a TAB tape. As a result of non-uniform stress in a carrier film, the TAB tape becomes deformed, making device fabrication difficult. Copper and polyimide, which make-up most TAB tapes, have different thermal expansion rates over various temperature ranges, including those temperatures experienced during device operation. Changes in temperature cause regions A, B, C, and D to expand and contract at different rates than other regions of carrier film 12 which have overlying copper electrical leads. Differences in expansion and contraction are explained by the fact that copper often has a coefficient of thermal expansion (CTE) which is different than CTEs of polymers used to form the carrier film. Because copper expands and contracts at a different rate for a given temperature change than does the carrier film, stress is created in the TAB tape which leads to deformation of the tape. FIG. 2 illustrates an example of a cross-sectional view taken along line 2--2 of TAB tape 10 in FIG. 1. As illustrated, carrier film 12 is deformed much in the way the film would deform in actual use, albeit the illustration is somewhat exaggerated for purposes of clearly understanding the problem. In practice, portions of the carrier film having overlying leads 20 may become convex relative to the upper surface of the film, while bare regions of the film, such as region D, may become concave upon experiencing a rise in temperature. Expansion rates for a given material vary with temperature; therefore, the deformation of carrier film 12 may be different than that illustrated in FIG. 2. For example, region D may become convex while portions of the carrier film having overlying leads 20 may become concave. Furthermore, the type of copper and polyimide used in a TAB tape will also affect the behavior of carrier film 12 during temperature changes.
Deformation of a TAB tape due to temperature fluctuations creates at least two problems in semiconductor devices. The first problem is related to assembly of a TAB semiconductor device, while the second is related to reliability of the device once assembled. In assembly of a TAB semiconductor device, deformation of the film causes leads 20 to become non-coplanar. Non-coplanarity of leads makes it difficult to properly bond the leads to either an electronic component, in the case of inner lead bonding (ILB), or to external circuitry, in the case of outer lead bonding (OLB). In bonding leads which are non-coplanar, the bonding process requires added operator attention and is very slow, often resulting in low device yield. Furthermore, non-coplanar leads often result in a misalignment of leads once the leads are bonded to bonding pads of an electronic component or external circuitry. From a device reliability point of view, deformation of a TAB tape within a finished TAB device may negatively impact device performance. A carrier film in a device may want to deform to relieve stress caused by a rise or fall in temperature within the semiconductor device or the surrounding environment. As the film deforms, bonds between leads and bonding pads of, for example, an integrated circuit or a printed circuit board may become broken, resulting in open circuits.
There are numerous factors which affect the degree and type of deformation of a TAB tape and, therefore, which affect TAB device performance. As mentioned previously, one factor is the mismatch of CTE values for various components in a TAB device. For the temperature range of interest in typical electronic systems (0.degree.-100.degree. C.), approximate CTE values for major components are as follows: the CTE of copper is in the range of 5-22 ppm (parts per million)/.degree.C.; the CTE of polyimide films and adhesives commonly used in TAB devices range from 8-16 ppm,/.degree.C.; and the CTE of a common electronic component, a silicon die, is about 2.5-3.0 ppm/.degree.C. As is evident from these values, the degree CTE mismatch between component can vary substantially. Furthermore the orientation of a component may also affect the expansion rates of the various components. For example, the CTE of copper and that CTE of a polyimide film can vary in the `X` and `Y` dimensions of the film (the film processing direction and the direction perpendicular to the processing direction, respectively). Such anisotropic behavior of a material's CTE is caused by preferred orientation of microstructural phases or regions in the films, such as copper crystals and polyimide molecules.
Yet another factor affecting TAB device performance is the ability of a TAB tape to accommodate stresses caused by temperature changes. The modulus of elasticity of commonly used polyimide films is quite high, while the thickness of most TAB polyimide films is 25-125 .mu.m. A combination of the polyimide film's high strength, stiffness, and thickness causes the polyimide to transfer some of the stress induced in the polyimide during temperature changes to much thinner copper leads. This stress transfer results in straining of the copper. If the strain exceeds the elastic limit of the copper, permanent plastic deformation occurs, thereby creating dislocations in the copper. Temperature cycling causes the dislocations to precipitate voids in the highly strained regions, leading to copper cracking which is also known in the art as fatigue failure. Cracked copper leads, if severe enough, will cause open circuits in the device or in a system application. Fatigue failure of copper leads is especially a problem in leads which are bonded to corners of an electronic component, such as an integrated circuit or silicon die. Fatigue failure is further accelerated by the fact that deformation of a TAB tape, particularly the portions of the tape nearest corners of the electronic component, adds a torsional component to the stress exerted on the corner leads. Also, inner lead portions of a copper lead are more susceptible to fatigue failure because inner lead portions have smaller lead dimensions than outer lead portions. Susceptibility to failure of inner lead portions is also due to the fact that inner lead portions are exposed to higher temperatures than outer lead portions since the inner lead portions are closest to an electronic component.
In view of the foregoing discussion, and more particularly, in view of the deficiencies and problems of existing TAB semiconductor devices presented above, a need exists for an improved TAB semiconductor device which has a reduced amount of deformation in a TAB tape or carrier film upon exposure to temperature changes. Further, a need exists for a method of making such an improved TAB semiconductor device.